Comparison of FSO 10 micrometer availability with FSO 850 nm/RF hybrid network availability F. Nadeem, E. Leitgeb, M.S. Awan, M. Loeschnig Institute of Broadband Communication, Technical University of Graz, Graz, Austria farukh.nadeem@student.tugraz.at G. Kandus IJS Ljubljana, Slovenia Abstract—Free space Optic (FSO) can provide line of sight, wireless, high bandwidth multi giga bits per second links to fulfil high data rate requirements of future communication applications. However, the widespread growth of FSO has been hampered by availability and reliability issues. FSO links are highly weather dependent and attenuating factor like fog causes reduced link availability for considerable amount of time. The alternate solution is to use fail over with RF back up links that are less susceptible to weather attenuations. Another approach is to use longer wavelengths like 10 μm as these wavelengths have high penetration against fog. In this paper, the availability of hybrid FSO/RF network has been compared with the availability of 10 μm under different weather attenuating condition like fog, rain and snow. The availability analysis has been performed using measured attenuation data. Keywords- FSO, RF, Fog, Rain, Snow, Availability, Simulation I. INTRODUCTION FSO features of secure, easily deployable, high data rate, license free and low bit error rate alternate to RF wireless communication links have led interest in this field. Some of the envisaged FSO applications are delay free web browsing, electronic commerce, data library access, streaming audio and video, video on demand, video teleconferencing, real time medical imaging transfer, enterprise networking, work-sharing capabilities and high speed interplanetary internet links [1]. The propagation of FSO through atmosphere has been influenced by weather condition like fog, rain and snow. Among various atmospheric attenuation effects on FSO link, fog is the most important factor that causes significant attenuation for considerable amount of time [2]. Earlier results have shown that attenuation has peak values of 130 dB/km in continental fog conditions in Graz (Austria) and 480 dB/km in maritime fog conditions in La Turbie (France) [3]. This makes the carrier class availability 99.999% a difficult goal to achieve. The addition of back up link can circumvent the issue of reduced FSO availability. However it requires switching back and forth between two applications. Another approach is to use longer wavelengths 10 μm that has fog penetration as high as twice (stable fogs) and ten times (selective fogs) the transmission of shorter wavelength like 850 nm and 950 nm [4]. Moreover it has been found that 10 μm wavelength has 29 dB advantage over 1550 nm against solar radiation [5]. Moreover the scintillation effects have been found to be inversely proportional to wavelength [6] which further advocates for longer wavelengths of 10 μm. In addition to that longer wavelengths can transmit more than an order of magnitude more power than similarly configured systems operating at 1.55μm, and still maintain the highest level safety classification [5]. The features like higher fog penetration of longer wavelengths motivates to compare the availability of longer wavelengths alone and FSO/RF hybrid network. In this paper the availability of FSO/RF hybrid network has been compared with the availability of 10 μm for different weather conditions of fog, rain and snow. The measured attenuation data have been used for simulation and analysis. The remainder of this paper has been organized as follows: Section II describes the fog effects on hybrid wireless network and 10 μm. Section III presents rain effects for these links, whereas in section IV snow effects for these links are discussed. Measurement setup for availability analysis is presented in section V. Results and simulation are presented in section VI. Concluding remarks finalize this paper in section VII. II. THE FOG ATTENUATION OF HYBRID NETWORK Fog is the most important factor among various atmospheric attenuation effects on FSO communication. Mie Scattering theory provides the most accurate way to calculate attenuation in case of fog droplets. However, this calculation requires detailed information of fog parameters like particle size, refractive index, particle size distribution etc. which may not be readily available at a particular location of installation. Moreover it involves complex computations. Alternate way is to predict specific attenuation due to fog using visibility data. The models proposed by Kruse, Kim and Al Nabulsi [7, 8, 9, 10] use visibility to predict specific attenuation. The small liquid water droplets of fog also cause scattering and significant attenuation for backup radio frequencies higher than 10 GHz. The effect on frequencies less than 10 GHz is ignorable [4]. The specific attenuation for GHz frequencies is given as [11] ( ) () 1 / km dB M K l c = γ 978-1-4244-2515-0/09/$25.00 ©2009 IEEE